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The present invention generally relates to an apparatus and method for detecting and calculating ground fault resistance. In a more specific embodiment, the present invention relates to an apparatus and method for calculating ground fault resistance, including an estimate of ground fault location.
It is common practice in the industry to isolate the field of a generator from ground. Excitation systems supplied for these applications are also isolated from ground and have traditionally included a field ground detector to detect the presence of a ground and to either generate an alarm or trip the generator. Indeed, the field ground detector has become standard in most exciter applications. The presence of one ground in the field or associated excitation equipment does not pose a problem, but the presence of a second ground can cause large currents to flow, which may cause damage to the equipment. Examples of applications pertinent to this invention are excitation systems and regulator systems in generator applications.
Historically, ground detectors have measured leakage current and have included some provision to ensure that a ground occurring anywhere in the field circuit can be detected, e.g., to ensure that there are no xe2x80x9cblind spots.xe2x80x9d Further, these devices are required to operate even when the field is not energized. These features resulted in devices that applied a continuous voltage from one field lead to ground. This produced the negative consequence of raising the field structure above ground and causing the sensitivity to vary as a function of applied field voltage.
In other applications, a low frequency oscillator has been used to replace the continuous voltage to ground applied to one field lead. The oscillator modulates the center of the field by a small voltage above and below ground. This solution eliminates the elevation in voltage of one field lead above ground. However, this solution has shortcomings. For instance, this solution does not eliminate the equipment""s variable sensitivity as a function of applied field voltage. Further, present implementations may operate in an incorrect manner in the presence of certain ground faults.
Further, it is desirable that the field ground detector operate correctly in the presence of high common mode voltages, and switching voltages associated with a Thyristor Bridge. Present implementations suffer from variable sensitivity and incorrect operation in the presence of certain ground faults.
Accordingly, there is a need for a more efficient apparatus and method for calculating ground fault resistance, including an estimate of ground fault location. There is a further need to monitor the ground resistance to detect degradation of ground insulation.
The solution described herein satisfies the above-described needs, as well as additional needs.
One exemplary purpose of the solution described herein is to detect a ground fault that occurs anywhere in the field circuit and in any of the exciter circuits galvanically connected to the field. A further exemplary purpose of the solution is to be able to identify the ground resistance so that it can be monitored to detect gradual degradation of the ground resistance. A further exemplary purpose is to provide the ability to estimate the location of the ground fault when the excitation system is in operation.
The field ground detection arrangement of the present invention has the ability to estimate the actual resistance of the ground fault and to further estimate the location of the ground fault. The invention can operate in the presence of high common mode voltages, rapid switching events and large capacitors to ground from each field lead. In one manner, the invention utilizes a low frequency square wave oscillator to permit measurement of the ground fault resistance when field voltage is not applied, to insure that there are no blind spots when the field is energized, and to provide a method for estimating the ground fault resistance.
The field ground detector can detect a fault essentially anywhere in the field circuit and in the power components associated with the excitation system. Ground faults that occur on the AC side of the Thyristor Bridge will be identified as AC ground faults for fault resistances less than 1500 ohms. Fault resistances greater than this value will be correctly identified but will not distinguish the fault as being on the AC side of the bridge. The field ground detector incorporates redundant techniques to increase the reliability and has provisions for measuring the low frequency oscillator voltage as a means of increasing the accuracy of the ground fault resistance calculation.
The low frequency oscillator voltage is, in one manner, measured at every transition in voltage levels, whereby much of the ground detector is tested every half cycle resulting in enhanced diagnostics. A xe2x80x9ctestxe2x80x9d function may also be supplied to force the low frequency oscillator to go to a much higher frequency during the test and measures the signature of the resulting signal. Thereby providing a more complete functional test of the ground detector and performs properly even in the presence of a preexisting ground fault.
As discussed above, the solution of providing a low frequency oscillator that essentially modulates the center of the field a small voltage above and below ground does not eliminate the variable sensitivity with applied field voltage. One aspect of the present invention utilizes a low frequency oscillator to avoid biasing one of the field leads above or below ground and removes the variable sensitivity of the field ground detector with field voltage.
Static excitation systems generally require the addition of a filter circuit, commonly known as a shaft voltage suppressor, to avoid coupling rapidly changing AC voltages into the shaft. These filters are traditionally implemented as RC circuits from each field lead to ground and the field ground detector has to operate correctly with these devices in place. The field ground detector of the present invention utilizes the response of the shaft voltage suppressor to fault resistances on the AC side of the bridge structure to identify the fault as being on the AC side of the bridge. This inventive aspect further provides a method for using redundant techniques to enhance the reliability of the field ground detector when used with redundant excitation systems.
In summary, one objective of the invention is to detect a ground fault that occurs anywhere in the field circuit and in any of the exciter circuits galvanically connected to the field. A further objective is to identify the ground resistance to effectively monitor to detect gradual degradation of the ground resistance. A further objective is to estimate the location of the ground fault when the excitation system is in operation. To bolster reliability objectives, the ground detector may utilize redundancy concepts. It is a further object to provide diagnostics to manage, monitor and maintain the operation and efficacy of the field ground fault detector of the present invention.
In one embodiment, the present invention provides a system for detecting field ground, comprising a sense resistor, an attenuator network, a reference signal source, a voltage controlled oscillator, and control logic. The reference signal source provides a reference signal to field leads of a generator. The reference signal is operatively coupled with the sense resistor and an attenuator network, wherein the attenuator network is coupled to both field leads resulting in a signal that biases the center of the field plus and minus the reference signal with respect to ground. The voltage controlled oscillator (VCO) measures differential voltage across the sense resistor. And the control logic coordinates the collection of measurement data, and extracts an estimate of ground fault resistance and location of a ground fault from the measured data.
In yet another embodiment, the present invention provides a method for use in a control system for operating an electrical machine. The method involves detecting and measuring a field ground fault and comprises the steps of: sensing the resistance of a sense resistor; applying a reference signal to field leads of the generator through the sense resistor and attenuator network, wherein the attenuator network is coupled to both field leads resulting in a signal that biases the center of the field within a range of plus and minus the reference signal with respect to ground; measuring the differential voltage across the sense resistor; analyzing the collected measurement data; and detecting a ground fault and estimating ground fault resistance and location of a ground fault from the measured data.
Still further features, purposes and advantages of the present invention are identified in the ensuing description, with reference to the drawings identified below.